Modern wireless communication systems employ spectrally efficient, digitally modulated signals with wide bandwidths and time-varying envelopes. Variations in the envelope magnitude of digital modulations generate distortion components at the output of the transmitter that are caused by the inherent nonlinearity of RF amplification circuits. Amplifier distortion produces a dilation of the spectrum of the input signal (“spectral regrowth”) which causes interference to communications in adjacent channels. Adjacent channel interference (ACI) is a highly undesirable phenomenon that is tightly controlled by regulatory organizations (FCC, ETSI, ITU). In addition to spectral regrowth, amplifier nonlinearities produce in-band distortion (i.e. distortion components within the bandwidth of the modulated input signal) which deteriorates the integrity of the transmitted signal and results in high Bit Error Rate (BER) at the receiver end. Nonlinearities in conventional RF amplifiers (Class AB) are relatively minor (distortion approximately 40 dB below the level of the carrier for output back offs (OBO) equal to the peak-to-average power ratio (PAR) of the modulation). The price to pay for such a mild nonlinear behavior is poor efficiency, i.e. limited DC-to-RF power conversion performance. High efficiency amplification is highly desirable since it improves system reliability (longer mean time before failure (MTBF)), simplifies thermal management, reduces amplifier size (lower silicon requirements) and lowers the operational and ownership costs of base stations. High efficiency amplifiers (e.g., Doherty amplifier designs) on the other hand exhibit much more nonlinear behavior than Class AB designs (distortion at or less than 29 dB below the level of the carrier).
One simple method of linearization increases the level of OBO in order to reduce output distortion by extending the linear range of operation of the amplifier. This technique can be successfully applied to enhance the linearity of Class AB amplifiers. Unfortunately it also produces a severe loss in efficiency due to reductions in RF output power resulting from higher OBO levels. High efficiency amplifiers on the other hand cannot typically be linearized by simply increasing OBO and require more sophisticated linearization techniques. A well-established technique uses Cartesian (or polar) feedback to minimize the output distortion of the amplifier. Feedback linearization can be effective for narrow signal bandwidths but has very limited distortion correction capabilities when wide bandwidth operation is required (e.g. multicarrier WCDMA) due to input-output stability restrictions associated to loop dynamics. Therefore, feedback would not be sufficient to linearize wide bandwidth, highly efficient transmitters. Another technique, feed forward, is based on additive post-correction of amplifier distortion, typically employing a dual loop architecture to estimate the output distortion of the amplifier in a first (carrier cancellation) loop and then injecting the distortion estimate, properly phased and scaled, to the output of the “main” amplifier via an RF auxiliary or “error” amplifier in a second (distortion cancellation) loop. Feed forward linearization systems do not suffer from the bandwidth limitations of feedback linearizers and are unconditionally stable.
However, and despite having wide bandwidth distortion correction capabilities, feed forward systems usually have low efficiency due to the DC power consumption of the error amplifier and the presence of lossy delay elements at the output of the main amplifier required for phase alignment of the distortion cancellation loop.
An alternative technique suitable for high linearity and high efficiency amplification is predistortion linearization. Conceptually and as a first order approximation, the predistortion technique linearizes the amplifier by injecting a compensatory distortion component at the input of the amplifier whose phase is opposite (180 degrees out of phase) to that of the amplifier's output distortion and whose amplitude is that of the output distortion divided by the linear gain of the amplifier. Predistortion does not suffer from the stability and severe bandwidth restrictions of feedback linearization systems. It also has the advantage over back off and feed forward linearization that its application in a well designed system does not result in a severe degradation of amplifier efficiency. Due to these inherent advantages, predistortion linearization has been the subject of intense research over the past decade.
Prior approaches to predistortion linearization have primarily focused on the design and implementation of digital LUT (Look Up Table) predistorters given the flexibility, precision and noise immunity advantages that they typically offer in comparison to analog predistorters. In these LUT based systems predistortion is carried out in baseband in either polar or Cartesian coordinates. In polar digital predistortion systems a conversion between Cartesian/polar coordinates is usually necessary due to the fact that the digital input modulation is in quadrature form. The coefficients of the predistorter are adaptively computed and stored in tables indexed by transformations of the input (or output) signal envelopes. Typically such LUT predistorters are intended to only compensate for nonlinear static amplifier distortion, without provisions for the linearization of dynamic nonlinearities in the amplifier. These “static” predistorters are not well suited for high efficiency base station transmitter designs due to the fact that nonlinear dynamic distortion components or “memory effects” constitute a substantial portion of the total output distortion of high power, high efficiency amplifiers. To address this limitation of LUT predistorters nonparametric digital baseband predistorters have been proposed in which multidimensional tables are indexed by dynamic transformations (filtered versions) of the instantaneous input envelope magnitude or power. The main advantage of the multitable technique for nonlinear dynamic distortion compensation is that it does not require the computation of a parametric model of the inverse dynamics of the amplifier. The main disadvantages are large memory requirements for storing the predistortion coefficients and the computational complexity involved in the interpolation of table entries when there is unreliable/insufficient data for system adaptation. Multitable interpolation complexity can be quite substantial, posing a limit to the accuracy and adaptation rate of the predistorter. The accuracy of digital LUT predistorters is also limited by table quantization errors. Quantization errors can severely limit distortion correction in high efficiency, high power transmitters in which wideband dynamic distortion compensation is necessary to meet stringent emissions specifications. A simple way to minimize table quantization error would be to increase table size. This solution is a viable alternative for some applications employing parametric digital LUT predistorters. Unfortunately increasing the number of table entries in multitable designs is prohibitive due to the rapid increase in memory and computational requirements. To improve the accuracy and lower the complexity of LUT predistorters a number of other predistortion systems have been proposed. However, none of these approaches adequately addresses the above problems.
The present invention is directed to overcoming the above noted shortcomings of the prior art and providing a predistortion system suitable for wide bandwidth applications without introducing undue complexity into the system.